Optical measuring systems that probe test objects with relatively movable measuring beams generally include a beam source, optics for shaping the measuring beam, a scanning device for moving the measuring beam in conjunction with one or more motion stages often under control of a programmed controller for relatively adjusting the position of the measuring beam with respect to a test object, a detector for detecting changes in the measuring beam associated with encounters of the measuring beam with the test object, and a processor programmed for interpreting the detected changes in the measuring beam as measurements of the test object. The measuring beams as optical probes are often arranged to collect data in discrete amounts over limited focus spot sizes on the test object approaching the measurement of individual points on the test object. Such point-by-point measurements are generally recorded within a common coordinate system in which measurements obtained with the measuring beam are combined with measurements also recording the movements of the measuring beam relative to the test object.
Overall, the measuring beams are scanned over predefined areas of the test object. Individual measurements are often collected during momentary lapses in the scanning motion during which relative motion between the measuring beam and the test object is discontinued. Each discrete measurement generally takes place over a finite exposure period during which sufficient quantities of light are collected to support subsequent processing of the collected data. The start-stop stepwise displacement profile of the measuring beam together with the required exposure time for taking measurements while the measuring beam is stopped, significantly extend the time required to take measurements over prescribed areas of test objects.
The stepwise scanning motion of the measuring beam with respect to the test object involves accelerations and decelerations that can momentarily destabilize the relative position of the measuring beam with respect to the test object. To avoid undesirably expanding the confined areas over which the discrete measurements are made, additional time may be needed to allow the relative position of the measuring beam to stabilize at each desired measuring position before measurement data is collected.